The modern automobile is being increasingly equipped with various accessories, some of which make driving easier and offer passenger comfort. Accessories can include the engine fan, alternator for the battery, air conditioning, and pumps for the engine cooling fluid and for servo-operations such as automatic transmission operation, power steering, power brakes and the like. Accessories are designed to operate at high and top engine speeds; however, most accessories have an optimum operating speed for a given condition. At low engine speeds the loss of efficiency or power by operating accessories is generally low. At high speeds, the loss of horsepower because of the operations of the accessories is much greater.
Accessory drive systems have been suggested which respond only to engine speed; which change the drive to a lower accessory ratio with increasing engine rpm, thereby maintaining accessory speeds essentially uniform over a large portion of the driving speed of the vehicle. Such drive systems are not generally sensitive to the demand or output of functions of the accessories, nor are they sensitive to the demand placed upon the vehicle engine. For example, in an accessory drive system responsive to speed of the engine, the power drain by the accessories can make acceleration of the vehicle sluggish or nonexistent.
Current stock accessory drive systems can be designed for adequate output for the worst operating conditions. For example, the engine fan must spin sufficiently fast to cool an air conditioning equipped vehicle at idle on a 95.degree. F. day. Thus, the fan is rotated much faster than is actually required for cooling for the average conditions. It thus consumes more power--and fuel--than is generally necessary. Generally all the accessories share the characteristic of actually wasting energy to provide adequate output for extreme conditions. The ideal accessory drive system then can be envisioned as being a combination of separate power packs for each accessory, each individually controllable to provide ideal operation of the accessory. One can imagine the added cost for such system and also the space requirement for such system when the trend is toward more compact and lighter systems in smaller spaces.